Server-side injection - Digital Guardian Agent X-dg headers cause 552 5.3.4 error

On Windows machines with Digital Guardian Agent installed, emails sent using Outlook can have numerous x-dg-ref headers added containing email and attachment classification information.

This can cause emails with large numbers of attachments to exceed the header size limit of Exchange.

When sending using server-side injection, Microsoft apply differing header size limitations to inbound and outbound connectors. This can cause emails to be correctly delivered to the Signature 365 servers from Exchange online, but cause Exchange to fail receipt when returned from the service with the following error:

As a result the Signature 365 service is unable to relay emails back to Exchange, and an NDR is delivered to the sender detailing this fact.

To resolve this issue, the article below from Fortra explains how to update the configuration settings within Digital Guardian to prevent these x-dg headers from being added to emails. Due to the hard size limit on email headers within Exchange, this must be completed in order to guarantee delivery of email when using server-side injection.

Digital Guardian Agent X Headers on Emails Causes Mail Server Rejection